1. Field of the Invention
The present invention relates to transverse axial tomographic systems using a computer and particularly to a single case on which a number of X-ray tubes and a plurality of detectors are carried.
2. Description of the Prior Art
Computerized tomographic apparatus at first included a source emitting a fine beam of X-rays in the direction of a slice of the body to be examined, and a detector positioned to receive the rays and to measure their intensity after they had passed through the body. The source-detector combination was submitted to a rectangular movement perpendicular to the direction of the beam, then to a rotation through a small angle around an axis perpendicular to the plane being examined, then again to a rectangular movement, and thus continuing until the combination had turned approximately 180.degree..
Such a prior art system took too long a time to produce the image of a slice of an organ in motion, such as the heart, where a time quicker than approximately 0.1 seconds is required.
To achieve faster operation, mechanical movement (translation, rotation) of the sources of X-rays and the detectors must be avoided, or at least reduced to a minimum.
In certain tomographic systems, the rectangular sweeping movements have been eliminated. The source then emits a beam of X-rays which is fan-shaped, with a large angle in the plane of the slice which transverses the body and simultaneously irradiates several detectors that are positioned one next to the other on this plane. The source-detector combination is thus only submitted to a rotational movement around the body.
In other systems, the mechanical translational movements of the source are replaced by a sweeping of the electronic beam of the X-ray tube on the target emitting the X-rays, which represents the length of translational movement.
But in all of these alternatives the time required for the tomographic picture is still long because of the mechanical rotational movement of the X-ray tube.
Several prior art patents have recognized the advantages of avoiding mechanical rotational movements. They use X-ray tubes where the tube is in the form of a toroid of very large diameter. However, the manufacture of such tubes is, in the current state of the art, very difficult and exacting.
In another patent, there is suggested the use of several tubes of the type providing a fan-shaped beam of X-rays placed successively in a circle, at the center of which is placed the body to be examined. The detectors are placed between the tubes in such a manner as to be irradiated by the tube, which is diametrically opposite. Thus, each fan-shaped beam produced by a tube irradiates the detectors placed between the two tubes which it faces.
With this arrangement, however, simultaneous irradiation of a body of normal dimensions does not include a sufficient number of directions for a tomographic slice. The opening of the fan-shaped beam of one tube is limited by the corresponding number of detectors, i.e., by the spacing between two tubes. Since this must be satisfied, the number of tubes must be small.
To remedy this drawback, it has been proposed to rotate the assembly through an angle of 2.pi./n (n=number of X-ray tubes) in small steps, in such a manner as to multiply the number of directions of measurement and of absorption. The number of tubes being small, the angle of rotation is large, and the time for acquiring the reading will be, again, too long to take a tomograph of an organ in motion. Moreover, each tube must include a protective sheath and a starting system; this arrangement appears very bulky and poorly adapted for a rotation.
The object of the invention is a tomographic apparatus having none of these inconveniences and permitting the taking of tomographic slices very rapidly.
Towards this end, the translational movement has been avoided and the rotational movement simplified and reduced to a minimum, permitting a very rapid tomograph.
According to one aspect of the invention, there is a casing in the shape of a toroid or half-toroid whose average diameter, for example, is one meter, and on which X-ray tubes are positioned one next to the other. The beams of these X-ray tubes are fan-shaped. The body to be observed is placed at the center of this toroid. The detectors cover its surface facing the body in such a manner as to measure the absorption in a very large number of directions in a given plane, more or less simultaneously (when the tubes are functioning one after the other or in groups).
According to other features of the invention, the casing may also contain grid systems for the cathodes, high-voltage starting circuits for the tubes, as well as isolated interfaces of the high voltage assuring a direct connection between a commutation circuit allowing a computer to stop or to start the tubes and a starting circuit.Thus, the bulk of the very large apparatus of high voltage isolation necessary in the apparatus having several tubes of the just-described prior art is avoided.